The present invention relates to the automatic calibration and adjustment of medical diagnostic imaging equipment. It finds particular application in conjunction with balancing photomultiplier vacuum tubes used in conjunction with Anger or nuclear cameras and will be described with particular reference thereto.
An Anger or nuclear camera commonly includes a scintillation crystal which emits flashes of light or scintillations in response to incident radiation. Typically, the scintillation crystal is on the order of 20 to 40 cm. by 40 to 80 cm. A multiplicity of vacuum photomultiplier tubes are arranged in a close packed hexagonal array. Typically, the Anger or nuclear camera includes a few dozen photomultiplier tubes. Each photomultiplier tube responds to each scintillation within its field of view by producing an electrical pulse. Typically, each scintillation is viewed directly by a plurality of surrounding photomultiplier tubes and indirectly as scattered light by more distant tubes. Each photomultiplier tube responds to each viewed scintillation by producing an electrical pulse whose magnitude is proportional to the brightness of the scintillation. The relative magnitude of the signals from the photomultiplier tubes viewing a scintillation is indicative of the relative distance of the scintillation from each photomultiplier tube. The sum of the magnitudes from all tubes viewing a common scintillation is indicative of the energy of the radiation. The electrical pulses from the photomultiplier tubes are processed, as known in the art, to provide an indication of the coordinates on the scintillation crystal at which each scintillation occurred and to reconstruct this information into medical diagnostic images.
It is to be appreciated that the magnitude of the electrical pulse signals from the photomultiplier tubes is determined not only by the distance of the scintillation from the photomultiplier tube and the energy of the radiation, but also by the gain of the photomultiplier tube. During manufacturing, a matched set of vacuum tubes are selected for each camera, i.e., vacuum tubes which have substantially the same gain. During manufacturing and initial calibration, the gain of the photomultiplier tubes is precisely adjusted or balanced individually. However, photomultiplier tubes, like other vacuum tubes, undergo a change in characteristics with use, e.g., an age related reduction in gain. Some vacuum tubes, known as "flyers", have a gain which increases with age rather than decreasing. The change in characteristics varies from tube to tube such that the set of photomultiplier tubes that were precisely balanced during manufacturing need periodic rebalancing.
Typically, Anger or nuclear cameras undergo a daily start-up procedure which checks various calibrations and adjustments of the camera. One part of this procedure includes to place a uniform flood source, e.g., a radioactive isotope vial or sheet in a position where it irradiates the entire surface of the scintillation crystal uniformly. The photomultiplier tubes view the scintillations and generate a resultant image. If the camera is in proper adjustment, the resultant uniform flood image is a uniform image of constant color and intensity. Variations in the color or intensity are indicative of various adjustment and calibration errors in the camera. Errors in the relative gain of the photomultiplier tubes manifest themselves in bright spots under tubes whose gain is higher than the other tubes and dark spots under tubes whose gain is lower than the other tubes. Typically, the photomultiplier tube gain can become several percent out of balance before there is a human-noticeable difference in the intensity of the uniform flood image.
Various techniques have been developed for calibrating the photomultiplier tube gain. In one technique, an LED of fixed intensity is mounted adjacent the photomultiplier tubes to bounce light off or through the crystal. The high voltage supplied to each photomultiplier tube is adjusted to adjust their relative gain. See, for example, U.S. Pat. No. 4,605,856. One drawback of this technique is that variations in the optical coupling between the LED and the scintillation crystal cause errors in the resultant gain adjustment. In a related technique, the LED is incorporated into each photomultiplier tube where the optical coupling is more highly controlled. Incorporating the LED in the photomultiplier tube is expensive. Moreover, the light output of LEDs is temperature variant.
In another technique, an aperture plate is disposed between the uniform flood source and the scintillation crystal. One of the holes in the plate is centered on each of the photomultiplier tubes. The photomultiplier tubes are individually selected to count radiation events for a preselected duration. Each tube is then adjusted until the counts are substantially uniform. See, for example, U.S. Pat. No. 4,517,460. One of the difficulties of this technique is that a very large count is required for statistical accuracy. This requires a test fixture and a long calibration time on the order of hours.
In another technique, the output of the photomultiplier tubes is monitored whenever the camera is in operation to determine a continuously updated energy response vector. The energy response vector is multiplied by a deconvolution matrix to deconvolve the contribution of adjoining tubes. The deconvolution matrix is the inversion of a contribution matrix containing matrix elements which represent the relative contribution of each radiation detector of the system. See, for example, U.S. Pat. No. 4,583,187. One drawback to this technique is that the gain adjustment is always on. Adjustments in gain can be made during a patient examination and are subject to statistical variations. Adjustments vary with conditions. Another drawback is that all nearest neighbors contribute. There is no individual tube selection.
The present invention contemplates a new and improved photomultiplier tube gain adjustment technique which overcomes the above-referenced problems and others.